I think Larry, W7IUV remarked about not liking manually tuned.. tuned
inputs.... as having to have yet
one more thing to tweak, when changing bands etc.
Using BOTH hands, it takes 45-50 seconds to switch from 15m.. to 160m [worse
case] on a
YU-148 amp... and that's including the KW big band switch, BOTH vac tune +
vac load caps..
and also the tuned input band switch.. and also BOTH broadcast air variables
used for the tuned input.
Both vac caps use turns counters.. and each individual revolution is split
into 100 x increments [ typ 3" skirt
has 100 x increments] The 2 x broadcast variables both use a large 4"
skirted Jackson bros infamous
6:1 reduction drive. The JB ball drive skirts are calibrated 0-100.. over
a 180 deg arc.
Having the PI net tuned input totally independent [ not ganged] from the
hi power PI net, allows you to
do cool things. Like, if you want to run 1/2 power.. 1/2 drive, etc....
normally the loaded Q would double.
If on 40m,.. simply throw the hi power band switch to 80m.. and loaded Q is
back to normal. Meanwhile
the tuned input is not affected at all.
Some variations of the manually tuned.. tuned input include using a small 4 uh
roller coil. Other
configs I use include a tapped 4 uh coil [6-8 ga wire... 17 turns on a 1.5"
ID].. suspended between
2 x 2240 pf broadcast caps. [ 4 x section, 17-540 pf per section, all 4 x
sections strapped in parallel]
I found if the loaded Q was too high..[like 5-6] ..esp on the high bands..
that the tapped 4 uh coil would
run warm on 17 +15+10m. Since the GG tube is very close to 50 ohms... we
used coax between output
of tuned input... and cathode. For testing, we put bird line sections on each
side of the tuned input....then
into a heath cantenna dummy load. With 200 w cxr... we were only getting
150- 160 w into the dummy load...
on the high bands! By increasing the coil uh... by JUST 1/4 turn... power
into dummy load shot up to 195 watts.
This of course, makes a helluva difference when driving any GG amp. Now the
drive to the cathode is way up, more
grid current for XXX drive, and more power output, etc.
With the manually tuned input, once tweaked for each band.... you just 'dial
by the numbers' after that... it's that
simple. With the low Q.. + 6:1 JB ball drives and 0-100 scale across a
4" skirt [ I also use the smaller 2" skirts]
it's repeatable every time. Flip band switch, tweak caps to pre-sets.. bam,
dead flat input swr.
One other major advantage.... the length of coax from xcvr to input of amp is
now a moot point. We tried everything from
2' up to 85'.. and in-between.. makes no difference. A dead flat input swr
can always be obtained. [ not like
my L4B's.. where > 3'.. and swr rises] .
The original idea came from Ron VE7XR.... who got it from Orr's very last
book. Orr had used a manual tuned
input.. but only for 160 + 80m monoband amps. Ron tried the 2 x broadcast
caps [ fair radio.. $16.00] and
a tapped coil on his 2 x 4-1000 amp. [80-10m] It worked superb. It also
works good on any other GG tube.
The small band switch I use.. is usually either a 2 x wafer.. or a 3 x wafer
unit [1.5 to 2" diam, small ceramic, SSON]
2nd wafer just pads the broadcast cap on 160M. 160m position of 1st wafer
pads the 2nd cap. 1st wafer does the
coil taps. [160m not tapped.. the entire coil is used.. freeing up the 160m
position for the padder] The 3rd wafer
activates the plate choke relay. I use a 2 x piece 135-180 + 45-70uh plate
choke set up. I used 4 x 500 pf
HT50/58 caps to pad each air cap on 160m.
Just for a laugh..... [with the dummy load set up and 2 x line sections] we
rammed 800 w cxr through the tuned input
on EACH band for 10 mins... and it runs just fine.. no heating. Moot point
anyway.. since in normal operation, the tuned
input is subjected to XXX cfm from the blower. It's a simple setup VS 9 x
tuned inputs... and also cover's 1.8 to 30 mhz
continuously. My patience ran out a long time ago, with trying to build 9 x
tuned inputs. The 2 x caps + tapped coil
do take up space though.. which was not an issue on the hb amps.
later........... Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|